1. Field of the Invention
The invention is directed to compositions and methods for removing residual oil from oil field produced water; and more particularly, the invention is directed to a method of removing residual oil from oil field produced waters using water-soluble polymers dispersed in a concentrated salt media.
2. Description of the Prior Art
As oil field reservoirs age and become depleted, one method to increase oil production is to maintain the pressure in the formation by injecting water or steam into the formation. The water or steam forces the oil out of the formation and to the surface. This method for sustaining oil production is referred to as secondary oil recovery. Secondary recovering is one of the most widely used recovery methods.
In secondary oil recovery, the produced fluids include the injected water emulsified with the oil. In order for the oil to be sold, it must first be separated from the water. The oil separation process is, however, not totally efficient. Some amount (200-10,000 ppm) of oil remains emulsified in the produced water. It is this waste water which is of concern. The produced water must be treated in some manner to remove the residual oil before discharge.
The emulsified oil in the produced water is typically present in the range of several hundred to tens of thousands of ppm. It is critical to remove this residual oil not only from an economic standpoint of selling the oil, but also from an environmental standpoint. The United States Environmental Protection Agency has placed tight restrictions on total oil and grease (TOG) limits for water that is to be discharged into public drinking water supplies or into open bodies of water. In addition to the governmental regulations, the residual oil must be removed in order to maintain a clean source of water or steam for reinjection into the underground formation. Failure to do so would result in eventual plugging of the formation and decreased production.
One of the most effective methods of removing the emulsified oil is through the use of the chemical aids. Historically, dry polymers, solution polymers, inverse emulsion latexes and metal ions have been used to treat the produced water. Each material has its own advantages and disadvantages.
While dry polymers have the benefit of being extremely concentrated, thereby reducing shipping costs, the equipment to dissolve the polymers is expensive and is not available in the oil field.
Latex polymers also suffer from problems but are used quite frequently. Latex polymer preparations include 30-35% solids dispersed in oil. The latex polymer must be also inverted prior to use. Like the dry polymer, the equipment in the oil field for prior inversion is not available, thus forcing the direct feeding of the polymer into the system. Numerous problems associated with this feeding method have caused many customers to avoid latex polymers. In addition, the latexes generally have a very narrow treating range, often resulting in over-treatment at higher dosages.
Although solution polymers require no prior make up, the percent solids and molecular weight are severely limited due to the nature of the material. These materials are often used to break reverse emulsions, but they are unable to flocculate the dispersed oil, thus requiring another chemical to accomplish this.
Metal ions, such as, Fe.sup.3+, Zn.sup.2+, Al.sup.3+, etc., have long been used to break reverse emulsions, but recent government regulations have restricted their levels in discharged streams. Although effective at breaking reverse emulsions, they, too, require another chemical to flocculate the oil.
The water-soluble dispersion polymers of the invention offer many solutions to these problems and represent a new method to treat oil field produced waste waters.